In larger devices, such as transmission facilities, exchange devices and wireless base stations etc., the modules have direct or indirect connection, so it is necessary to communicate between the modules frequently. The communication information includes not only those for normal service, but also those for maintenance, such as alarm, fault detection etc.
At present, there are two ways for communication between modules in devices:
The first way is to communicate through a mailbox, as shown in FIG. 1. The control module 101 is connected with every module 102 through the data bus, address bus and control bus, and because the mailbox can bi-directional communicate, the control module 101 can make communication with every module 102.
The second way is shown in FIG. 2. The control module 201 is connected with every module 202 through the 485 buses, and the communication can be implemented through the 485 buses between the module 201 and every module 202, or between different modules of 202.
Although the bus mode of the above two ways has a simple structure that is easy to implement, and supports various protocols, but there are several disadvantages as follow: the load capacity of a bus is limited, so it does not support communication of many modules; if a fault happens at a point, it is easier that the whole bus cannot communicate or the communication reliability become worse, and also it is difficult to locate the fault point; furthermore, since there are many connection lines, the backplane circuit design and layout are complex.